Metcalf boxed his way to a decision victory, with the Liverpool fighter winning 96-95, 96-94 and 97-94 on all three of the judges' scorecards.
Those figures don't take into account a knockdown in round three, which the referee judges as a slip.
Lejarraga wasn't at his absolute best, despite the vocal backing of his home supporters in Bilbao.
Speaking after the fight, Metcalf told Matchroom Boxing: "Can't put it into words, I'm not a man of many words anyway. But I'm made up."
Asked whether he was surprised in any way by Lejarraga's strategy, the Liverpool-based fighter said: "Not really no, he's usually similar in every other fight.
"(He) does what he does well, he's strong, he's got a great jab. He lets them go when he's inside. He came up with a plan and I nullified it.
"He's a very tough person. He's a big puncher. Every jab he hit me with I could feel. Every other shot as well. He's very tough."
Looking ahead to his next challenge, the 33-year-old said: "Before this fight, they kept talking about fighting for the WBA title. Whatever they come up with, I'll take. I'll fight anyone, anywhere. Whatever Eddie (Hearn) fancies, I'll go with him."
Promoter Eddie Hearn was thrilled with Metcalf's victory, saying: "It was a great performance, really disciplined. I was trying to score it like I was in Bilbao and was a Kerman Lejarraga fan and I still had JJ winning 6-4.
"It was probably wider than that, but sometimes you don't get the decisions out here. And that was the worry.
"It would've been a big injustice if he didn't get the win tonight and he deserves it. He's always stepped up for the fights. Jumped in with Ted Cheeseman in a brilliant fight - a fight of the year candidate.
"Boxed Kieron Conway in a fight that was very close as well. We gave him a run out to get himself ready for this fight and he came-in in tremendous shape, you could see that as soon as I saw him in the changing room, I knew he was ready.
"It was a very intelligent performance and the team. They knew Lejarraga, they studied him. Obviously, he's done many rounds with him. They put the game plan in place to be victorious."
Looking at the next step, Hearn added: "I think big fight. He deserves his big night and his big money. I'd like to think we've looked after him along the way but he want the big one and as much money as possible, every fighter wants that.
"People that have continuously stepped up and taken the challenges, deserve to get that opportunity when they win.
"The great thing about these cards is, although we're trying to find emerging talent from Spain and Italy, we're also giving British fighters the opportunity - it's up to them. And they can change their career and turn their career around in these kind of nights."